One of the post translational modifications, N-linked glycosylation, of the insulin receptor has been studied in cultured cells using an experimental approach based on mutagenesis of the insulin receptor cDNA at specific sites of potential modification, transfection of the cDNA stably into cultured cells, and then study of these receptors structurally and functionally. Processing of these mutants was investigated by biosynthetic labeling, and isolation of the insulin receptor at various time points. Receptors unable to glycosylate their cells in the first four potential glycosylation sites have abnormal processing. These receptors do not appear on the cell surface and remain in proreceptor form in the endoplasmic reticulum. Mutants of each of these sites are being made in order to determine the significance of each of these sites to the normal processing and intracellular transport of the receptor.